1. Field of the Invention
The present invention mainly relates to an optical recording medium such as an optical disk, and also to an optical recording apparatus which uses the optical recording medium.
2. Description of the Related Art
Various types of optical disk apparatuses are known, each can record data by applying an intense light beam such as a laser beam onto the concentric circular tracks or a spiral track formed on an optical disk, and also can read or reproduce the data from the optical disk by applying a less intense light beam onto the concentric tracks or the spiral track. The optical disk apparatuses are classified into three types. The first type is write once (read many) type, the second type is rewritable-type, and the third type is read only type.
In recent years, the write once type optical disk apparatus has been developed and put on the market before the rewritable-type apparatus. With this optical disk apparatus the user can write data on the disk, in addition to the data recorded on the disk by the manufacturer of the disk. The write once type apparatus was first made commercially available in the form of a document filing system. Now it is also available in the form of a peripheral memory device of computer for highly reliability, and the like. The rewritable-type apparatus can erase data from an optical disk and can write new data thereon.
The technique employed in the optical disk apparatus of either type, for recording and reproducing data, is performed also in an optical card apparatus for recording data on an optical card and reproducing it therefrom, and in an optical tape apparatus for recording data on optical tape and reproducing it therefrom. Hereinafter, optical disks, optical cards, and optical tape will be generally called "optical data-recording medium," and any apparatus for recording data on, and reproducing it from, an optical disk, an optical card, or optical tape will be generally called "optical data-recording/reproducing apparatus."
The concentric circular tracks or the turns of the spiral track of most commercially available optical disks are spaced apart at intervals of about 1.6 .mu.m. To record data on the track, a converged laser beam is applied from a laser onto the track, forming a light spot having a diameter of about 1.2 .mu.m on the track, and forming a record mark having a diameter of about 1 .mu.m in the track. The record mark is formed by destroying or deforming that portion of the recording layer of the disk which has been illuminated with the laser beam. Alternatively, the mark is formed by changing the optical property of said portion of the recording layer.
The optical disk has a groove extending along the spiral track or grooves extending along the concentric tracks, respectively. An optical disk apparatus has a tracking control system comprising a multi-segment photo-detector and a device for controlling the position of the laser spot. The optical detector receives the laser beam reflected from any groove and generates a tracking error signal from this beam. The error signal is input to the position control device. In accordance with the error signal, the device moves the objective lens to such a position that the laser-beam spot lands fully on the target track.
The tracking control system has a drawback. When the disk is placed eccentric to the spindle of the disk drive, the target track deviates from the axis of the objective lens which converges the laser beam. When the disk warps or inclines, the beam reflected from the target track deviates from the optical axis of the optical detector. In either case, the tracking error signal has the offset which corresponds to the deviation of the track or the reflected beam. The offset contained in the error signal results in an inaccurate tracking control. Hence, the laser beam applied via the objective lens forms a light spot on the disk, which is deviated a little from the target track. Consequently, data cannot be correctly recorded on, or reproduced from, the target track, and the optical disk apparatus is less reliable than required.
To eliminate the drawback of the tracking control system, a new tracking control technique has been developed. This technique is characterized in two respects. First, an optical disk is used which has elongated mirror-surface areas extending in the radial direction of the disk and mutually spaced apart, and discontinuous guide grooves extending along the spiral track or concentric tracks and located among the mirror-surface areas. Second, the laser beam reflected from any mirror-surface area is converted into an electric signal, and this signal corrects the tracking error signal generated from the laser beam reflected from the guide groove adjacent to the mirror-surface area. The optical disk, for example, a write once type disk having a diameter of 130 mm, is pre-formatted such that it has a number of sectors, and each sector has one mirror-surface area.
FIG. 7 is an enlarged plan view of the optical disk of the type described in the preceding paragraph. As is shown in this figure, guide grooves 401 extend along the concentric tracks or the spiral track, prepits 403 are formed in each track, and a mirror-surface area 404 extends in the radial direction of the disk. The concentric tracks or the turns of the spiral track are spaced apart from one another, at intervals of 1.6 .mu.m. The grooves 401 and the prepits 403 have been formed at the time of manufacturing the substrate of the disk. The prepits 403 are used as sector marks, as marks for generating clock-sync signals, or as marks representing address data.
FIG. 8 is a graph explaining how the levels of two tracking error signals change with time, which are being generated from a beam reflected from the optical disk shown in FIG. 7. Plotted on the horizontal axis is the tracking error, i.e., the distance between the center line of the target track and the center of the light spot the converged laser beam forms on the disk. Plotted on the vertical axis is the level of either tracking error signal.
Curve 501 designates an error signal generated from a laser beam reflected from the disk when the target track does not deviate from the axis of the objective lens, nor the warps or inclines. As can be understood from the curve 501, the error signal is at zero level when there are no tracking errors. The operating point 502 of feedback control for setting the error signal 501 at zero level is located at the origin (0, 0). As a result, excellent tracking control can be achieved in accordance with the tracking error signal 501.
On the other hand, signal curve 503 designates a tracking error signal generated from a laser beam reflected from the disk when the objective lens deviates from the axis of the optical head, or when the disk has static deflection or tilt. As is evident from the curve 503, this error signal is not at zero level when there are no tracking errors errors. good tracking control cannot be accomplished in accordance with this tracking error signal curve 503. To achieve excellent tracking control, it is necessary to evaluate the offset 505 of the error signal curve 503 (i.e., the level which the error signal has when the tracking error is zero), and to perform correcting of the error signal and perform feedback control by the value corresponding to this offset 505.
When the objective lens deviates from the axis of the optical head, or when the disk warps or inclines, the tracking error signal generated from the laser beam reflected from any mirror-surface area of the disk has an offset, too. This offset is substantially proportional to, or is a simple function of, the offset 505 of the error signal generated from any guide groove 401. Hence, the offset of the tracking error signal generated from the beam reflected from the groove 401 can be compensated to a little extent in accordance with the tracking error signal generated from the beam reflected from the mirror-surface area 404.
In order to increase the reliability of the optical disk apparatus or the recording density, it is required that tracking control be achieved with higher precision. However, no high-precision tracking control can be accomplished by the above-described technique, wherein the offset of a tracking error signal generated from a laser beam reflected from the guide groove is compensated in accordance with a tracking error signal generated from a beam reflected from a mirror-surface area adjacent to the mirror-surface area. This is because the offset cannot be detected with a sufficiently high precision.
The tracking control technique, explained above with reference to FIGS. 7 and 8, is disadvantageous. The mirror-surface areas 404 may cause errors in counting the tracks which the laser beam crosses as the optical head is moved in the radial direction of the optical disk to the desired track. Track-counting errors, if any, can make a prominent bar to an increase in data-accessing speed of the optical disk apparatus.
In summary, the conventional technique of correcting the offset of a tracking error signal generated from a laser beam reflected from the guide groove adjacent to the mirror-surface area in accordance with a tracking error signal generated from a beam reflected from a mirror-surface area is disadvantageous in two respects. First it is hard to accomplish high-precision tracking control since the offset cannot be detected with a sufficiently high accuracy. Second, the mirror-surface areas may cause errors in counting the tracks scanned by the laser beam emitted from the optical head moving in the radial direction of the optical disk to the desired track, inevitably rendering high-precision access control impossible.